U.S. patent number 4,188,431 [Application Number 05/616,197] was granted by the patent office on 1980-02-12 for latent image printing and development.
This patent grant is currently assigned to The Gillette Company. Invention is credited to Robert F. Farmer, Phillip E. Sokol.
United States Patent |
4,188,431 |
Sokol , et al. |
February 12, 1980 |
**Please see images for:
( Certificate of Correction ) ** |
Latent image printing and development
Abstract
This invention relates to alkali developable, paper-based
systems printed with invisible phenolphthalein inks and made
reusable through many development cycles by employing critical
concentrations of dye reactants in conjunction with a paper having
a defined water soluble acidity-alkalinity.
Inventors: |
Sokol; Phillip E. (Rockville,
MD), Farmer; Robert F. (Gaithersburg, MD) |
Assignee: |
The Gillette Company (Boston,
MA)
|
Family
ID: |
24468421 |
Appl.
No.: |
05/616,197 |
Filed: |
September 24, 1975 |
Current U.S.
Class: |
428/29; 106/31.2;
106/31.43; 427/145; 428/199; 428/211.1; 428/537.5; 434/328 |
Current CPC
Class: |
B41M
3/001 (20130101); C09D 11/50 (20130101); G09B
3/04 (20130101); Y10T 428/31993 (20150401); Y10T
428/24934 (20150115); Y10T 428/24835 (20150115) |
Current International
Class: |
B41M
3/00 (20060101); C09D 11/00 (20060101); G09B
3/04 (20060101); G09B 3/00 (20060101); D21H
005/10 (); B44F 001/10 () |
Field of
Search: |
;428/29,199,411,537
;35/9G ;427/145 ;106/21 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1956143 |
|
Jul 1970 |
|
DE |
|
713025 |
|
Oct 1931 |
|
FR |
|
Other References
Milton Lesser, "Invisible or Sympathetic Inks", American Ink Maker,
Nov. 1945, pp. 27-30..
|
Primary Examiner: Robinson; Ellis P.
Attorney, Agent or Firm: Wise; Richard A. Janowski; Leonard
J.
Claims
What is claimed is:
1. A reusable paper-based feedback system comprising a paper
substrate having a water solubility acidity-alkalinity of from
0.08% acid as sulfur trioxide to 0.05% base as sodium hydroxide,
said paper substrate bearing
a. a visible image and
b. an invisible image derived from printing on said paper substrate
an ink comprising from 0.1 to 9.0% by weight phenolphthalein.
2. A reusable paper-based feedback system according to claim 1
wherein said paper substrate has a yellowness index of 20 to
50.
3. A reusable paper-based feedback system according to claim 1
wherein the paper substrate has a water soluble acidity of no more
than 0.06% and wherein the invisible image is derived from printing
on said paper substrate an ink comprising from 0.5 to 5.0% by
weight phenolphthalein.
4. A reusable paper-based feedback system according to claim 3
wherein said paper substrate has a yellowness index of 20 to
30.
5. A reusable paper-based feedback system according to claim 1
comprising, in addition, an alkaline developing composition in the
form of an aqueous solution of a compound selected from the group
consisting of monoethanolamine, propanolamine and water soluble
carbonate, phosphate, borate and silicate salts.
Description
FIELD OF THE INVENTION
This invention relates to materials and methods used in
informational, testing, educational, amusement and other
paper-based feedback systems embodying invisible imges comprising a
color-forming reactant material which can be rendered temporarily
visible by the application of a coreactant. The systems may also
include visible printed images and are suitable for multiple
reuse.
BACKGROUND OF THE INVENTION
Recent commercial applications in the art of latent image printing
and development have related primarily to teaching aids and
self-instructional material in which, for example, a printed
question or problem is presented along with a multiple choice of
answers, only one of which is correct. Associated with one or more
of the answers is an invisibly printed indication of its
correctness, adapted to show a color change when contacted with a
marking instrument containing a chemical component adapted to react
with the invisible ink. These and other applications of latent
image printing and development are disclosed in U.S. Pat. Nos.
3,363,336; 3,363,337; 3,363,338; 3,451,143; and 3,516,177.
A wide variety of invisible but color producing systems have been
proposed for such uses. The oldest known invisible inks are those
responding to heat. Such inks are invisible when applied to a paper
substrate but upon application of heat become visible. Examples
described in the literature include solutions of ferric sulfate
plus phosphoric acid, cobalt choride, alloxanthine plus oxalic
acid, and potassium nitrate. The use of onion and lemon juices as
heat sensitive, invisible inks is also disclosed in the prior
art.
Another type of invisible ink, the light sensitive ink, has usually
been based on dilute solutions of silver salts such as silver
nitrate, although other metal salts such as gold trichloride have
been used. The use of oxalomolybdric acid has also been reported.
Light sensitive inks appear to have found little commercial
utilization as invisible inks and no method of adapting such
systems to paper-based applications is apparent.
Another class of invisible inks includes those developed by contact
with water and consisting of a waterproof, transparent and
colorless substance which is applied to a colored or tinted paper
substrate. When the substrate is immersed or dampened with water,
the area surrounding the inks change color and the writing becomes
visible. On drying, the system reverts back to its original
invisible state. Ink systems which may be used include wax
solutions, various gums, metallic soaps and other similar, water
insoluble substances. While such systems have found wide usage in
advertising, a major limitation of such systems is the dependence
on the speed of solvent evaporation for reversion to the invisible
state.
A relatively new type of invisible ink is one which gives off a
bright fluorescence under the influence of ultraviolet light. Many
materials are reported which show such fluoresence but most are
unsuitable for use in invisible ink systems due to color or
solubility problems. Compounds which have been used in fluorescent
invisible inks systems include quinine sulfate, esculin,
anthracene, and alpha-phenyl-m-aminobenzothiazole
hydrochloride.
The class of invisible inks which appears to be best suited to
practical commercial application comprises chemically activated
inks based on the reaction of two relatively colorless materials to
form a colored compound. A wide variety of such chemical systems is
disclosed in the prior art and reference may be made to, inter
alia, U.S. Patent No. 3,363,336 and 3,438,927 and German Published
patent application 1,956,143. To be economically suitable for
commercial applications, chemically activated invisible ink systems
must be nontoxic and safe for use by children, printable using
existing printing equipment, provide a vivid color reaction, and
should be reusable several times with the paper substrate quickly
reverting back to its original state after each use. We have found
that invisible inks based upon phenolphthalein, printed on types of
paper as defined herein and developed with certain dilute alkaline
compositions fulfill the requirements given above.
We are aware that phenolphthalein-alkaline developer systems have
long been disclosed in the prior art in, for example, U.S. Patent
Nos. 456,047 and 866,293 and in German Published patent application
No. 1,956,143. To date, however, the art has apparently not
developed such a system capable of repeatedly being rendered
visible with the developed color automatically fading each time
within a controllably predetermined period of time.
DETAILED DESCRIPTION OF THE INVENTION
We have disclosed that alkali developable, paper-based systems
printed with invisible phenolphthalein inks can be made reusable
through many development cycles if certain critical concentrations
of dye reactants are employed in conjunction with a paper having a
specified water soluble acidity-alkalinity as more fully defined
hereafter. By proper choice of parameters, we have found it
possible to produce invisibly printed systems which upon
development will exhibit a visible mark for only a fraction of a
second or for several days before fading. At one extreme, very
short development periods are desirable in novelty items where a
single item is designed for multiple use by several members in a
group. At the other extreme, a period of visible development of
several days would be desirable in educational testing materials
where the developed answers are to be reviewed or evaluated by a
teacher several days after an examination is administered.
There are four main factors that control the color fading time of
the phenolphthalein printed, paper-based systems of this invention:
(1) the concentration of the phenolphthalein in the invisible ink
composition used to print on the paper substrate, (2) the type and
concentration of the alkaline reactant solution used to develop the
red phenolphthalein color, (3) the water soluble acidity-alkalinity
of the paper substrate, and (4) the number of development cycles to
which the printed paper has previously been subjected.
We have found that when the phenolphthalein ink comprises a simple
solution of phenolphthalein in ethanol, a phenolphthalein
concentration range of 0.1 to 9.0 percent by weight will provide
color fading times ranging from one second to over forty-eight
hours depending upon the characteristics of the developing
composition and paper chosen. Such inks are suitable for packaging
in conventional porous tipped pens or felt tipped marking devices
and may be employed in printing with conventional flexographic
printing equipment. Phenolphthalein inks may also be printed using
conventional dry offset or letter press equipment. However, when
such processes are used, it is necessary to formulate a thickened
water soluble vehicle containing the phenolphthalein and a pigment
adapted to mask the visible printing traces due to the vehicle.
These techniques are well known to those skilled in the art of
printing ink information formulation and hence will not be further
discussed herein. In the form of either simple alcoholic solutions
or formulated inks, we prefer to employ 0.5 to 5.0 percent
phenolphthalein by weight.
While from the standpoint of color development alone, any alkaline
composition might be used to develop the phenolphthalein-based ink,
we have found that many alkalis cause a residual yellow
discoloration in the paper substrate thus making them unsuitable
for use in systems where multiple development cycles are
contemplated. Among the alkaline developing agents found to be
useful are aqueous solutions of monoethanolamine, propanolamine,
and water soluble carbonate, phosphate, borate, and silicate salts.
The developing agents may be applied to the phenolphthalein printed
substrate in any convenient manner including spraying, brushing,
rolling or by the use of conventional marking instruments in which
a wick made of felt or other porous medium is loaded with the
developing agent. The preferred developing agents are
monoethanolamine and sodium carbonate. We have found that solutions
of monoethanolamine ranging in concentration from 0.5 to 2.0% by
weight will provide color fading times of from one second to nearly
six hours depending upon the phenolphthalein concentration employed
and the water soluble acid or alkali content of the paper. In like
fashion, we have found that similar concentrations of sodium
carbonate will provide color fading times ranging from one second
to 48 hours. Should even longer color fading times be desired, it
is possible to use concentrations of the alkaline color developing
solution up to 5.0%.
A third factor which influences color fading time is the water
soluble acidity or alkalinity (hereinafter referred to as
"acidity-alkalinity") of the paper substrate employed as determined
by ASTM Procedure D548-41. The water soluble acidity-alkalinity of
the various paper substrates described in the examples below was
determined using this procedure as follows.
Fifteen grams of each paper was ground in a Wiley Mill equipped
with #20 screen. A 3-5 gram sample of each paper was weighed out to
the nearest milligram and placed into a 500 ml Erlenmeyer flask.
The flask was placed into a heated oil bath at 100.degree. C. and
350 ml of water added with stirring. At the end of one hour the
flask was removed from the oil bath, and the contents immediately
suction filtered through a coarse sintered glass filter. Fifty ml
of distilled water was used to wash each flask clear of any
remaining paper solution. The total filtrate was then allowed to
cool to room temperature. To the room temperature solution was
added 1 ml of 0.1% phenolphthalein solution, and the solution was
then titrated with either 0.01 N sodium hydroxide or 0.01 N
hydrochloric acid as required. Total water soluble
acidity-alkalinity content of each 5 gram sample was then
calculated as set forth in ASTM Procedure D-548-41. We have found
that a wide variety of commercially available papers including
groundwood, book or bulk book grade and regular offset grades of
paper may be used in the practice of our invention. While uncoated
papers can be used satisfactorily where lowest cost is desirable,
coated grades should be used where minimal feathering and gloss is
desired. In all cases, however, the paper should not have a water
soluble alkalinity, as determined above, greater than 0.05% base
(as sodium hydroxide). While it is possible to employ papers having
a water soluble acidity as determined above as high as 0.08% acid
(as sulfur trioxide), we have found that papers of high acid
content tend to exhibit increased yellow discoloration upon
repeated color development. For this reason, we prefer to employ
papers having a water soluble acidity of no more than 0.06%.
We have indicated above that the repeated application of aqueous
alkaline solutions to paper substrates can cause discoloration or
yellowing of the paper. While this can be minimized by proper
choice of alkaline developing solution and paper acidity, we have
found that the number of repeat development cycles possible with
any particular combination of paper and developer can be increased
by the use of a paper stock having a color carefully chosen to mask
any alkaline discoloration produced on the paper. We have found to
be useful those papers having a yellowness index, YI, ranging from
20 to 50 as defined in and measured by ASTM Method E313, Indexes of
Whiteness and Yellowness of Near-White Opaque Materials. We prefer
to use paper substrates having a yellowness index of 20 to 30.
The following nonlimiting examples illustrate the principle and
practice of this invention.
EXAMPLE I
The color fading times for a variety of combinations of paper type,
phenolphthalein ink concentration and aqueous monoethanolamine
developer concentration were determined by marking a sample of each
of the papers described below with a commercially available porous
point pen loaded with the phenolphthalein ink composition. After
drying for 5 minutes, the phenolphthalein mark was then crossed
with the monoethanolamine developer solution contained in a
commercially available felt tip marking pen. The elapsed time from
the application of the second reagent to the time the resulting
color completely disappeared is shown below as the fading time.
Each result is the average of three determinations.
__________________________________________________________________________
COLOR FADING TIMES FOR PHENOLPHTHALEIN-MONOETHANOLAMINE SYSTEMS %
Monoethanolamine Paper % Phenolphthalein 0.5 1.0 1.5 2.0
__________________________________________________________________________
Uncoated, bulking book 1.0 12 min 60 min 95 min 110 min grade paper
having a 3.0 14 min 85 min 135 min 170 min water soluble alkalinity
5.0 53 min 165 min 200 min 255 min of 0.017% 7.0 57 min 180 min 250
min 290 min 9.0 65 min 270 min 320 min 405 min Uncoated regular
offset 1.0 1 sec 23 sec 4 min 20 min grade paper having a water 3.0
4 sec 6.5 min 45 min 63 min soluble acidity of 0.02% 5.0 5 sec 25
min 105 min 150 min 7.0 6 sec 53 min 185 min 450 min 9.0 8 sec 85
min 200 min 585 min Uncoated groundwood book 1.0 0 sec 3 sec 7 sec
15 sec grade paper having a water 3.0 1 sec 4 sec 12 sec 30 sec
soluble acidity of 0.069% 5.0 1 sec 5 sec 15 sec 39 sec 7.0 1 sec 6
sec 23 sec 45 sec 9.0 2 sec 9 sec 35 sec 58 sec Uncoated bulking
book grade 1.0 1 sec 9 sec 27 sec 1 min paper having a water
soluble 3.0 1 sec 28 sec 4.5 min 7 min acidity of 0.059% 5.0 4 sec
40 sec 10 min 16 min 7.0 5 sec 1.5 min 23 min 32 min 9.0 5 sec 10
min 40 min 58 min
__________________________________________________________________________
EXAMPLE II
Following the procedure described in Example I, the color fading
times for a variety of combinations of paper type, phenolphthalein
ink concontration and aqueous sodium carbonate developer
concentration were determined with the following result.
__________________________________________________________________________
COLOR FADING TIMES FOR PHENOLPHTHALEIN-SODIUM CARBONATE SYSTEMS %
Sodium Carbonate Paper % Phenolphthalein 0.5 1.0 1.5 2.0
__________________________________________________________________________
Uncoated, bulking book 1.0 4 sec 90 min 11 hr 19 hr grade paper
having a 3.0 5 sec 135 min 16 hr 26 hr water soluble alkalinity 5.0
6 sec 3.5 hr 20 hr 31 hr of 0.017% 7.0 8 sec 11 hr 25 hr 36 hr 9.0
14 sec 19 hr 31 hr 48 hr Uncoated regular offset 1.0 2 sec 37 min
11 hr 19 hr grade paper having a water 3.0 3 sec 80 min 14 hr 20 hr
soluble acidity of 0.02% 5.0 4 sec 140 min 17 hr 29 hr 7.0 5 sec 10
hr 21 hr 35 hr 9.0 8 sec 16 hr 28 hr 44 hr Uncoated groundwood book
1.0 1 sec 1 sec 2 sec 6 hr grade paper having a water 3.0 1 sec 2
sec 3 sec 7 hr soluble acidity of 0.069% 5.0 2 sec 3 sec 4 sec 8 hr
7.0 3 sec 4 sec 5 sec 9 hr 9.0 4 sec 5 sec 4 hr 19 hr Uncoated,
bulking book grade 1.0 1 sec 2 sec 3 sec 25 sec paper having a
water soluble 3.0 2 sec 3 sec 5 sec 40 sec acidity of 0.059% 5.0 3
sec 5 sec 6 sec 11 hr 7.0 6 sec 7 sec 8 sec 16 hr 9.0 14 sec 23 sec
150 min 22 hr
__________________________________________________________________________
EXAMPLE III
Using the method described in Examples I and II, the color fading
times of systems employing 5% phenolphthalein and 1%
monoethanolamine with a variety of paper types were evaluated as a
function of the number of times the monoethanolamine developer was
applied to the same area of the phenolphthalein printed paper,
showing the relationship between changes in fading time and the
water soluble acidity-alkalinity of the paper employed.
______________________________________ No. of Paper Markovers
Fading Time ______________________________________ Uncoated,
bulking book grade 1 2.2 hr paper having a water soluble 2 3.7 hr
alkalinity of 0.07% 3 4.8 hr 4 5.3 hr 5 6.3 hr Uncoated regular
offset grade 1 1.0 hr having a water soluble acidity 2 2.7 hr of
0.02% 3 3.6 hr 4 4.7 hr 5 5.7 hr Uncoated, bulking book grade 1 40
sec paper having a water soluble 2 0.5 hr acidity of 0.059% 3 1.3
hr 4 1.9 hr 5 2.3 hr Uncoated groundwood book grade 1 5 sec having
a water soluble acidity 2 45 sec of 0.069% 3 150 sec 4 0.2 hr 5 0.5
hr ______________________________________
EXAMPLE IV
A conventional crossword puzzle of the type commonly found in
newspapers and magazines is printed in black ink on an uncoated
regular offset grade paper having a water soluble acidity of 0.02%
using flexographic printing equipment. The empty spaces within the
crossword puzzle itself are printed solidly with an ink comprising
5% solution of phenolphthalein in ethanol.
The user of the puzzle solves it in the conventional manner except
that in the place of the usual pen or pencil, there is employed a
porous point pen of the type commercially available but loaded with
a 1% aqueous solution of monoethanolamine. Wherever the pen
contacts the phenolphthalein printed portions of the puzzle, a
corresponding bright pink mark or line is generated. Within one
hour after completion of the puzzle, all of the answers which have
been marked in have totally faded permitting the puzzle to be used
again. Even after the fifth such repeated use, all of the marks
generated by contact between the phenolphthalein and
monoethanolamine fade within less than six hours.
By varying the choice of paper substrate, the concentration of
phenolphthalein ink employed, and the identity and concentration of
the alkaline developer, similar crossword puzzles can be produced
having color fading times ranging from instantaneous to several
days.
EXAMPLE V
A multiple choice examination dealing with questions of American
history is printed on uncoated, bulking book grade paper having a
water soluble alkalinity of 0.17% employing conventional black
printers ink and a two color dry offset printing press. Each of the
multiple choice answers is followed by a visibly printed box having
dimensions of approximately 1 by 2 centimeters. Printed invisibly
within each box using the ink formulation shown below is either the
word "right" or "wrong".
______________________________________ Ingredient % by Weight
______________________________________ phenolphthalein 0.97 ethanol
4.85 glycerin 7.77 white clay pigment 5.83 blown silica suspending
agent 2.91 triglycerol 77.67
______________________________________
In the instructions for taking the examination, the student is
directed to use a felt tipped marking pen containing a 2% aqueous
solution of sodium carbonate which is provided with the examination
and to make a mark in the single box following the answer which he
judges to be correct. The teacher administering the examination may
wait more than 8 hours before grading such an examination paper and
still see clearly the pink mark generated by the reaction between
the sodium carbonate and the phenolphthalein. After approximately
24 hours, however, all of the marks denoting the answers will have
completely faded, permitting the examinations to be reused by other
students. In using this type of examination, there is of course no
need to return the paper to the student after the grades are
recorded by the teacher since each student will be aware of the
number of his errors at the conclusion of the examination.
When a printed examination of the type described above is printed
on an uncoated, bulking book grade paper having a water soluble
alkalinity of 0.017% and a yellowness index, YI, (as hereinbefore
defined) of 18.0, visible alkaline staining occurs after four
markovers with 1.5 to 2.0% monoethanolamine developing solutions.
When a similar paper having a yellowness index of 25.2 is employed,
the phenolphthalein printed portion can be developed 12 times
before visible alkaline staining occurs.
* * * * *